1. Field of the Invention
The invention relates to a radiation CT (Computed Tomography) radiographing device for tomography photographing an object, a radiation CT radiographing system, and a radiation CT radiographing method using such a device and a system. More particularly, the invention relates to a cone beam radiation CT radiographing device using a 2-dimensional radiation image sensor panel. Further, the invention relates to a cone beam radiation CT radiographing device of an object rotating type by which an object such as an organism or the like in the state of standing on a floor or sitting on a chair is rotated and photographed between a radiation source and a radiation image sensor facing each other.
2. Related Background Art
In recent years, digitization has been progressed in various medical fields. Also in the X-ray diagnosis field, to digitize an image, there has been developed a 2-dimensional X-ray image pick-up device in which an incident X-ray is converted into visible light by a scintillator (phosphor) and the visible light image is further picked up by an image pick-up element.
The following advantages of the digital X-ray radiographing device over an analog photographing technique exist: elimination of films (filmless system), increase in information obtained by image processes, creation of a database, and the like.
In the field of an X-ray still image, as a 2-dimensional X-ray image pick-up device, for example, a still image pick-up device (flat panel detector) of a large plate of a size of 17 inches (43 cm×43 cm) using amorphous silicon (a-Si) has been manufactured as a device for photographing a breast and a chest region.
In the field of an X-ray motion image, the following device and system have also been proposed: a 2-dimensional digital X-ray radioscopic device in which an incident X-ray is converted into visible light by the scintillator (phosphor) and an I.I. (image intensifier) and the visible light image is picked up by a TV camera using a CCD type image pick-up element; and a system in which the TV camera is replaced by the flat panel detector.
Further, the flat panel detector can also be applied to the X-ray CT radiographing device. Japanese Patent Application Laid-open No. 2001-194461 discloses a technique in which the flat panel detector is applied to a general X-ray CT radiographing device in which an object lying on its side on a bed is photographed while an X-ray valve and a detector, which are arranged so as to face each other, are rotated around the object.
There has also been proposed a device in which a rotating plate that can rotate a human body in the standing or sitting state is provided in order to perform a lung cancer medical examination for a generally healthy person, and the human body is photographed while the rotating plate is rotated for an X-ray source and a 2-dimensional X-ray image pick-up device such as I. I. TV camera, flat panel detector, or the like. Such a device has been disclosed in Japanese Patent Application Laid-open No. H5-42132, Japanese Patent Application Laid-open No. 2000-217810, and the like. In FIG. 12, reference numeral 1 denotes an X-ray CT device as an example of such a device; 2 a scanner unit; 5 a rotating plate on which an object R is mounted; 6 an X-ray source; 7 an image intensifier (I. I.); 3 a processing circuit having a preliminary processing unit 8 and an image reconstructing unit 9; and 4 a display.
In the device, the photographing of the object can be promptly started in the state where the object is standing and can be replaced by the next object in a short time, so that the whole photographing time can be shortened. Further, since the photographing can be completed by at most one rotation by using a cone beam X-ray, radiation exposure dosage of the object can be reduced and the like. Therefore, such a device is preferable as a system suitable for a medical examination. Since a gantry to integratedly rotate the X-ray source and the X-ray detector is unnecessary, the positions of the X-ray source and the X-ray detector can be freely set, and an enlargement ratio upon measurement and a measuring region can be changed. Since the X-ray source for general photographing can be used and the gantry is unnecessary, the costs of the system can be suppressed. Further, since a construction of the system is simple, it is advantageous for vehicle-mounted use.
As a feature of a cone beam X-ray CT radiographing device which will be mentioned in the invention, a detector merely rotates on a rotary axis and is not moved in the rotary axial direction. Therefore, if there is a 2-dimensional detector of a large plate which can grasp the whole pertinent region of the object, all projection image data to reconstruct the whole CT image can be obtained by one rotation.
In the case where the conventional object rotating type cone beam X-ray CT radiographing device is used for a medical field, it is demanded to shorten restriction time of a human body serving as an object as much as possible. As a method of shortening a holding time of the same posture of the object, that is, a time that is required for measurement, there is a method of raising a rotating speed of the object. However, a time of about 5 seconds or longer is necessary for rotating the object once. There is a problem that if the object is rotated at a speed higher than such a rotating time, the object enters what is called a dizzy state where he feels dizzy.
There is also a problem that when the object is rotated at a high speed, a centrifugal force which is applied to the object also increases together with an increase in rotating speed, so that the object is moved during the measurement.
It may be necessary for the object being photographed to hold his breath during photographing depending on a photographing region. In the case of photographing a region where a body's movement due to a breath is large, particularly, upon photographing a chest region or the like, the breath must be held. There is also a problem that since the breath pausing time of about 5 seconds per rotation is relatively long, an examinee feels anxiety during the examination, a body movement artifact occurs due to an unexpected operation, body vibration, or the like, or the object is out of a field of vision of the detector, so that the photographing fails.
There is also a problem that if the object is moved during the measurement, “blur” or “deflection” occurs in a 3-dimensional X-ray distribution image formed by the reconstruction and picture quality of a 3-dimensional X-ray image deteriorates.
In the case of rotating the rotating plate on which the object with a considerable weight is mounted, it takes time until the rotating speed reaches a predetermined speed and becomes stable. There is, consequently, a problem that more than one rotation is necessary and a burden on the patient becomes heavy.
There is also a problem that if the photographing is started in the unstable state, rotational angles of respective projections differ, precision of the reconstruction deteriorates, a variation in a signal from a reference element increases, precision of the correction deteriorates, and the image deteriorates.
There is also a problem that, generally, in the X-ray CT radiographing device, even if the X-ray is radiated while a lamp voltage and a lamp current are set to the same conditions, upon photographing, X-ray intensity actually changes for every scanning, in detail, and in every viewing. This problem is caused by focal position changes or the like due to expansion/contraction of a shaft of an anode by a temperature change of an X-ray valve, movement of the shaft between bearings due to the motion of the valve, precession motion when the anode is rotating, and the like. As countermeasures against those problems, in the conventional single slice type X-ray CT radiographing device and the conventional multi slice type X-ray CT radiographing device, a method whereby a reference channel to measure the intensity of the X-ray which does not transmit the object in the X-ray from the X-ray valve is arranged in the edge portion of the X-ray detector, each channel value is corrected by using a value measured by the reference channel, and data to form an image is often used. However, there is also a similar problem in the object rotating type cone beam X-ray CT radiographing device.
There is also a problem that a complicated mechanism and control apparatus are necessary to rotate the rotating plate on which various objects are mounted while it is precisely controlled, so that the manufacturing costs rise.
There is also such a problem that in the case where the rotating plate on which the object is mounted is photographed while purposely changing the rotating speed in dependence on the photographing region of the object, an accurate image of each region cannot be obtained corresponding to each rotating speed set for every photographing region of the object.
Further, if the object is rotated by 360° for 5 seconds and photographed by the full scan by 1000 projections, images have to be read out for 5 msec per projection, that is, at a speed of 200 frames/sec. However, there is such a problem that it is very difficult to drive the flat panel detector using a thin film transistor of a-Si or a-Se with an area of the 17-inch size (43 cm×43 cm) or more at such a speed because semiconductor characteristics of a-Si or a-Se are inadequate.
There is such a problem that in the cone beam X-ray CT radiographing device in which an X-ray dosage per projection is fairly smaller than that of the still image, since noises are large and the sensitivity is low, the sensitivity in the conventional flat panel detector is insufficient.
The I. I. TV camera, which is advantageous in terms of the sensitivity, has problems peculiar to the image intensifier. That is, there are the following problems: an essential problem in which a distortion of the image occurs in the peripheral portion; a problem in which even in the current maximum 17-inch size, since the display screen is circular, an wide enough area almost equal to that of the flat panel detector cannot be obtained, the reconstructing area decreases remarkably in the cone beam X-ray CT; and a problem in which, since the magnetism is used for controlling an electron beam, if the device is rotated, it is complicatedly influenced by the terrestrial magnetism.
In Japanese Patent Application Laid-open No. H5-42132, nothing is considered with respect to countermeasures against the fluctuation of the X-ray source, rotational drift, and body movement in the case of rotating the object. In Japanese Patent Application Laid-open No. 2000-217810, only a measure against the body movement is taken and nothing is considered with respect to countermeasures against the fluctuation of the X-ray source and the rotational drift. Further, in Japanese Patent Application Laid-open No. 2001-194461, countermeasures against the reading speed and the sensitivity which are required in the object rotating type cone beam X-ray CT radiographing device are not disclosed.
Moreover, in Japanese Patent Application Laid-open No. 2003-66149, a multi slice X-ray CT radiographing device in which detectors having a plurality of resolution are arranged is disclosed.
The X-ray source for general photographing can be used in the object rotating type cone beam X-ray CT radiographing device. However, the following problems of the X-ray source exist.
That is, in an X-ray valve, an electron beam is radiated to a target so as to generate an X-ray. If a material of the target is tungsten, converting efficiency of an energy of the electron beam into the X-ray is less than 1% and most of the remaining portion of the energy changes into a thermal energy. Therefore, since a temperature of the target becomes high, a rotating anode is generally used as a countermeasure against such a problem.
The rotating anode has a conical shape and the electron collides with the side surface of the rotating anode. Although most of the heat generated in the target is radiated as a radiation energy to the circumference from the target, a part of the generated heat is radiated by thermal conduction through a rotary shaft. By repeating the X-ray radiation, a quantity of heat which is accumulated in the target in the X-ray tube increases and a temperature of the rotary shaft or the like rises. It has been known that thermal expansion is caused in the target by such a temperature increase.
A focal position is deviated by a positional deviation of the rotary electrode and the cathode and a positional deviation in the flying direction of the electron beam. Since such a phenomenon is caused due to a temperature change, aging change, abrasion of the electrode, or the like, it is impossible to avoid the situation wherein the focal position differs by a difference of the time position upon collection of X-ray absorption data, that is, by a difference of the X-ray exposing time point. In the case of the X-ray CT radiographing, a geometrical positional relation among the focal position of the X-ray source, the rotary shaft, and the detector is very important.
A fluctuation of sensitivity distribution of the detector due to the movement of the focal point becomes a calibration error resulting in an artifact on the reconstructed image. It is, therefore, necessary to correct the change in detection output of the X-ray detector due to such a deviation of the focal position. Therefore, a reference detector is prepared to monitor the X-ray intensity. For example, a reference element and correction in the general multi slice X-ray CT radiographing device have been disclosed in Japanese Patent Application Laid-open No. H8-154926. FIGS. 20A and 20B show such a conventional technique. In FIGS. 20A and 20B, reference numeral 25 denotes a reference detector as mentioned above; 23 an image detector; 24 a focal point; 27 an object; and 20 a collimator.
In the case where the object rotating type cone beam X-ray CT radiographing device is used for the medical examination, it is necessary to shorten the restriction time of the human body serving as an object as much as possible. A method of raising the rotating speed of the object can be mentioned as a method of shortening the holding period of the same posture of the object, that is, the time which is required for measurement. However, in the case of rotating the object at a high speed, the object enters what is called a dizzy state where he feels dizzy. It is, therefore, known that about 3 to 5 seconds are necessary as a time which is required for one rotation of the object.
Naturally, the X-ray has to be radiated from the X-ray source for such a period of time and the focus movement mentioned above becomes a problem. Moreover, since a countermeasure against the continuous photographing of a long period of time is not taken in the X-ray source for the general photographing, such a problem is more serious than that in the case of the X-ray source only for the X-ray CT radiographing. There is also such a problem that when the existing X-ray source is used, there are various histories and the change in age, abrasion of the electrode, and the like, which also differ with every device, so that it has to cope with them for every X-ray source. Further, since the X-ray source for the general photographing is not equipped with the foregoing reference detector, the reference detector has to be separately prepared and installed in the case of constructing the system.
On the other hand, according to the technique disclosed in Japanese Patent Application Laid-open No. 2003-66149, only the fixed resolution can be obtained. In Japanese Patent Application Laid-open No. 2000-217810, nothing is disclosed with respect to the problems which are caused by the focus movement of the X-ray source in the object rotating type cone beam X-ray CT radiographing device. In the detecting method of the focus movement in Japanese Patent Application Laid-open No. H8-154926, nothing is disclosed with respect to the precision of the beam detection and since substantially the same detector as the main detector is used, the beam can be detected only at the same column width as that of the main detector.